Insulation and airspace

I am replacing the roof of an existing house, structure and all, in Houston. I would like to do a standing seam metal roof. and exposed tongue and groove on the inside.
My question is what are your recommendations concerning the construction of this roof. I was thinking of beams, then joists, then the tongue and groove deck, then rigid insulation, then #30 felt paper, then red rosin, then the standing seam. The roof would have a continuous ridge beam and continuous soffit vent. I think I would need air space somewhere but don't know where.
I was very confused reading the various messages posted on the subject of insulation. In Houston, I think I would need at least a R40, but with rigid insulation, that would mean about 8" of it, mighty thick insulation. Plus the air space (how much?), that would make the roof very thick. Am I completely on the wrong track?

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You're not really on the wrong track but you are dealing with a tough situation.
I really think you need to have a structural engineer involved for the purpose of designing a good system for this and making sure the house can handle the weight of it.
To achieve eave to ridge ventilation, somehow you will need some vertical risers in the system. The ventilation should be on top of the insulation.
If the vertical risers are deeper than the insulation, then you can install either horizontal purlins or solid decking (as dictated by the roofing panel you choose) on top of them followed by the underlayment, red rosin paper, and metal roofing.
You might also want to consider some of the pre-fabricated stress skin "sandwich" panels for this application. I cannot vouch for their effectiveness but some of them have built-in ventilation channels.
Again, though, weightloads, etc. will need to be studied by someone experienced with such things.

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Thanks. A structural engineer is going to be involved as well as an architect. I found a material by Hunter called Cool-Vent. It looks like just what you had mentioned, a ventilated nailbase insulation panel.

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